The present invention relates to thermal recording (heat transfer recording) in which an improved thermal ink sheet or ribbon is repeatedly used for thermal recording. More particularly, the present invention relates to an improved thermal ink material.
As is well known, in a thermal recording process, an ink sheet, having a solid ink composition layer coated on a substrate such as a polyester film, is used. A thermal printing head contacts the substrate with pressure and transfers heat to the solid ink composition layer. The heat is selectively distributed onto the contact surface of the printing head corresponding to an image pattern to be reproduced. The heated solid ink material contained in the layer melts and adheres to the surface of a receiving sheet, i.e., printing paper, which contacts the ink sheet. Consequently, the image pattern is transferred onto the printing paper.
The thermal recording process is used in various information recording means, such as facsimiles, word processors, personal computers, automatic ticket issuing machines, etc. The thermal recording process has substantial advantages, such as low noise printing, compact size, low cost, and low running cost since plain paper is used as a receiving sheet. However, there are certain disadvantages.
In a prior art thermal process, a printed image is transferred onto a receiving sheet which is a smooth plain paper having a Bekk smoothness higher than 200 seconds. This is required in order to obtain clear printing quality. When using plain paper having a rough surface, the thermal solid ink material (hereinafter, simply referred to as "ink material") is required to increase its resinous components to strengthen the adhesion of the printed image onto the plain paper, or to improve its film forming ability and bridging property. As a result, the melting point of the thermal ink material rises, causing the printing ink sheet to have a lower printing sensitivity. Consequently, the printing energy is forced to be increased, resulting in a heavy load placed on the printing head and a short life thereof. Various counter measures have been employed including increasing the printing pressure of the thermal printing head onto the thermal ink sheet, improving the timing and peeling angle when the thermal ink sheet is peeled off the printing paper after the transfer of the printing image, and employing a printing head having an improved structure to achieve more sharp point contact between the printing head and the thermal ink sheet.
However, the biggest disadvantage of prior art thermal recording devices is the vast consumption of ink sheets. This is because for a single thermal recording step, all the ink material existing on the areas of the substrate of the ink sheet corresponding to the image pattern, is transferred, making it impossible to further use the ink sheet in a succeeding thermal recording step. An ink sheet of this type is referred to as "one-time" ink sheet, and is consumed each time the printing image is transferred to the printing paper. This increases the cost of the thermal recording process.
Recently, various types of reusable or "multi-time" ink sheets which can withstand repeated use have been developed to solve the above-described problems. Examples of "multi-time" ink sheets are disclosed, for example, in U.S. Pat. No. 3,392,042 issued on Jul. 9, 1968, to Hugh T. Findlay et al., Japanese Patent Laid-Open Provisional Application No. 57-160691 to Uchiyama et al., published on Oct. 4, 1982, and Japanese Patent Laid-Open Provisional Publication No. 58-183297 to Ohnishi et al., published on Oct. 26, 1983. Ink sheets in the above-identified references have a thermal ink composition layer, for example, a polyester film, disposed on a substrate. An ink composition layer contains a porous transfer layer therein, and ink material is contained in the pores of the porous transfer layer. When heat and pressure are applied to the ink sheet in the area corresponding to a recording pattern through contact with the associated printing head, the applied heat is transmitted through the substrate to raise the temperature of the ink material and to melt the ink material, thereby decreasing its viscosity. The heated ink material flows easily through the porous structure of the transfer layer, being expressed by the pressure applied toward the printing paper, and penetrating thereinto. The porous structure of the transfer layer provides flow resistance, limiting the quantity of the ink material which is expressed for a one time printinu. Thus, the thermal ink sheet is capable of being repeatedly used. Thereafter, the thermal printing ink sheet is peeled off the printing paper and the thermal recording process is completed.
The prior art reusable thermal printing ink sheet is three to five times thicker than that of a one-time thermal printing ink sheet. This is necessary in order to withstand the repeated thermal printing operation. However, this causes a decrease in the printing sensitivity of the device and requires an increase in the printing energy. Particularly, when a plain paper having a rough surface is used, more resinous ink material having a higher melting point and higher viscosity is required in the ink composition layer as described above. In a cold environment, therefore, such reusable thermal printing ink sheets are not usable in a conventional thermal printing apparatus.
Furthermore, prior art multi-time thermal printing ink sheets have other problems including background noise and the transfer of a ghost image onto the printing paper. The background noise is caused by the type of surface of the associated ink sheet. A powdery and adhesive surface of the ink sheet adversely affects printing. A printing paper is contaminated only by the friction between the surfaces of the relevant ink sheet and printing paper during a storage or printing operation. Accordingly, background noise is found in prior art "one-time" and "multi-time" ink sheets.
The ghost image is caused by the brittleness of the solid ink material. FIG. 1 has enlarged cross-sectional views of an ink sheet and plan views of images transferred onto a printing paper, illustrating the various states of the surfaces of the printing paper (upper row), the ink composition layer (middle row) and the cross-section of the ink composition layer (lower row), before printing, after printing, and during succeeding printing stages when printing pressure, but no printing signal, is applied (blank printing), respectively. As shown in the cross-sectional views, a substrate 1 is coated with an ink composition layer 3 which is a porous layer formed of coagulated carbon powders 4. Ink material 5, stored in the pores of the porous structure, contains low temperature melting compounds (waxes) and coloring agents. After an image is transferred onto the paper, an originally smooth surface of the ink layer 3 is roughened when the ink sheet is peeled off the printing paper 10. This results in micro-peaks 6 which are formed by half melted viscous ink material pulled in a direction normal to the surface of the ink sheet. The peaks 6 are distributed in the form of an inverse image 8.
If the ink material 5 is brittle in its solid state, that is, at room temperature, the peaks 6 are easily collapsed and tend to be transferred to the printing paper 10 when a printing pressure of the printing head (represented by a pressing roller 11) is applied to the ink sheet at a next printing step. As a result, a faint image 4 is left on the paper 10. The faint image 4 is referred to as a ghost image. If a new pattern is printed at a succeeding step, the new pattern image and the ghost image 4 are transferred onto the same portion of the printing paper 10. Thus, the printing quality is lowered. The background noise and the ghost image are found frequently in prior art thermal recording processes employing ink sheets which include low temperature melting compounds such as fatty acid compounds, fatty acid amide compounds, and ester compounds, because these materials are brittle or have powdery surfaces in their solid states.
Further, an ink sheet containing a low temperature melting compound of urethane having (NHCO) atomic bonding is proposed. The urethane compound features a narrow and sharply distinguishable melting temperature zone, resulting in a clearly printed image. On the other hand, thermal ink material containing a urethane compound has a strong adhesion force to paper and is substantially viscous near its melting temperature. When the ink sheet, including the urethane compound, is repeatedly used some background noise is present which is due to the high viscosity of the urethane compound. When the ink sheet is peeled off the printing paper just after the transfer of the printing image, the ink material is in a half melted state and leaves substantially sharp and elongated peaks of the solid ink material. Such elongated peaks collapse very easily, even though the urethane compound is not brittle, causing a ghost image on the printing paper. In addition, the resulting transferred image is a non-uniform printed image which has a rather low optical density and which has many white spots where no ink material is locally transferred. Thus, a non-uniform printed image which has a low optical density results. The non-uniform printed image is also due to the high viscosity of the ink material. Furthermore, the melting point is also high, requiring large printing energy and the use of relatively smooth plain paper as a receiving sheet.
Accordingly, an ink sheet containing a urethane compound is not suitable for a reusable multi-time ink sheet. Thus, a further improved thermal recording ink sheet has been expected in the field.